rails_admin with rails-api conflict

Question

I have a recently started project where i'm using mongoid for the database and rails_admin to manage it, now i want to have an api and i've choosed rails-api to do it, but for some reason since i just added rails-api to the gem file, i can't access to the admin and i'm getting always this error

NoMethodError in Devise::SessionsController#new

undefined method `[]' for nil:NilClass

and here is the trace

warden (1.1.1) lib/warden/session_serializer.rb:29:in fetch' warden (1.1.1) lib/warden/proxy.rb:196:inuser' warden (1.1.1) lib/warden/proxy.rb:293:in _perform_authentication' warden (1.1.1) lib/warden/proxy.rb:90:inauthenticate' warden (1.1.1) lib/warden/proxy.rb:100:in authenticate?' devise (2.1.0) app/controllers/devise_controller.rb:113:inrequire_no_authentication' activesupport (3.2.5) lib/active_support/callbacks.rb:407:in _run__3280997315660744487__process_action__4533000551616940901__callbacks' activesupport (3.2.5) lib/active_support/callbacks.rb:405:in__run_callback' activesupport (3.2.5) lib/active_support/callbacks.rb:385:in _run_process_action_callbacks' activesupport (3.2.5) lib/active_support/callbacks.rb:81:inrun_callbacks' actionpack (3.2.5) lib/abstract_controller/callbacks.rb:17:in process_action' actionpack (3.2.5) lib/action_controller/metal/rescue.rb:29:inprocess_action' actionpack (3.2.5) lib/action_controller/metal/instrumentation.rb:30:in block in process_action' activesupport (3.2.5) lib/active_support/notifications.rb:123:inblock in instrument' activesupport (3.2.5) lib/active_support/notifications/instrumenter.rb:20:in instrument' activesupport (3.2.5) lib/active_support/notifications.rb:123:ininstrument' actionpack (3.2.5) lib/action_controller/metal/instrumentation.rb:29:in process_action' actionpack (3.2.5) lib/action_controller/metal/params_wrapper.rb:206:inprocess_action' actionpack (3.2.5) lib/abstract_controller/base.rb:121:in process' actionpack (3.2.5) lib/abstract_controller/rendering.rb:45:inprocess' actionpack (3.2.5) lib/action_controller/metal.rb:203:in dispatch' actionpack (3.2.5) lib/action_controller/metal/rack_delegation.rb:14:indispatch' actionpack (3.2.5) lib/action_controller/metal.rb:246:in block in action' actionpack (3.2.5) lib/action_dispatch/routing/route_set.rb:73:incall' actionpack (3.2.5) lib/action_dispatch/routing/route_set.rb:73:in dispatch' actionpack (3.2.5) lib/action_dispatch/routing/route_set.rb:36:incall' actionpack (3.2.5) lib/action_dispatch/routing/mapper.rb:42:in call' journey (1.0.3) lib/journey/router.rb:68:inblock in call' journey (1.0.3) lib/journey/router.rb:56:in each' journey (1.0.3) lib/journey/router.rb:56:incall' actionpack (3.2.5) lib/action_dispatch/routing/route_set.rb:600:in call' rack-pjax (0.5.9) lib/rack/pjax.rb:12:incall' mongoid (2.4.10) lib/rack/mongoid/middleware/identity_map.rb:33:in block in call' mongoid (2.4.10) lib/mongoid.rb:133:inunit_of_work' mongoid (2.4.10) lib/rack/mongoid/middleware/identity_map.rb:33:in call' warden (1.1.1) lib/warden/manager.rb:35:inblock in call' warden (1.1.1) lib/warden/manager.rb:34:in catch' warden (1.1.1) lib/warden/manager.rb:34:incall' rack (1.4.1) lib/rack/etag.rb:23:in call' rack (1.4.1) lib/rack/conditionalget.rb:25:incall' actionpack (3.2.5) lib/action_dispatch/middleware/head.rb:14:in call' remotipart (1.0.2) lib/remotipart/middleware.rb:30:incall' actionpack (3.2.5) lib/action_dispatch/middleware/params_parser.rb:21:in call' actionpack (3.2.5) lib/action_dispatch/middleware/callbacks.rb:28:inblock in call' activesupport (3.2.5) lib/active_support/callbacks.rb:405:in _run__2911892753477349565__call__1112249857594481255__callbacks' activesupport (3.2.5) lib/active_support/callbacks.rb:405:in__run_callback' activesupport (3.2.5) lib/active_support/callbacks.rb:385:in _run_call_callbacks' activesupport (3.2.5) lib/active_support/callbacks.rb:81:inrun_callbacks' actionpack (3.2.5) lib/action_dispatch/middleware/callbacks.rb:27:in call' actionpack (3.2.5) lib/action_dispatch/middleware/reloader.rb:65:incall' actionpack (3.2.5) lib/action_dispatch/middleware/remote_ip.rb:31:in call' actionpack (3.2.5) lib/action_dispatch/middleware/debug_exceptions.rb:16:incall' actionpack (3.2.5) lib/action_dispatch/middleware/show_exceptions.rb:56:in call' railties (3.2.5) lib/rails/rack/logger.rb:26:incall_app' railties (3.2.5) lib/rails/rack/logger.rb:16:in call' actionpack (3.2.5) lib/action_dispatch/middleware/request_id.rb:22:incall' rack (1.4.1) lib/rack/runtime.rb:17:in call' activesupport (3.2.5) lib/active_support/cache/strategy/local_cache.rb:72:incall' rack (1.4.1) lib/rack/lock.rb:15:in call' actionpack (3.2.5) lib/action_dispatch/middleware/static.rb:62:incall' railties (3.2.5) lib/rails/engine.rb:479:in call' railties (3.2.5) lib/rails/application.rb:220:incall' rack (1.4.1) lib/rack/content_length.rb:14:in call' railties (3.2.5) lib/rails/rack/log_tailer.rb:17:incall' rack (1.4.1) lib/rack/handler/webrick.rb:59:in `service'

i really don't have idea what is happening between those gems that cause this, any advise or direction is welcome ;)

Answer 1

I went through the same problem and I found out that Warden needs two middlewares that doesn't come by default in an API application: ActionDispatch::Session::CookieStore and ActionDispatch::Cookies.

So, these middlewares can be added via:
#config/application.rb
config.middleware.use ActionDispatch::Cookies
config.middleware.use ActionDispatch::Session::CookieStore